Transmission line to waveguide mode transformer

ABSTRACT

An ultra broadband, low-loss, transmission line to waveguide mode transformer that can be implemented using PCB technology includes a dielectric substrate on which is disposed an electrically conductive pattern that forms a dielectric waveguide and a dielectric transmission line extending to the waveguide. The electrically conductive pattern includes a waveguide portion that forms the dielectric waveguide. It also includes a transmission line portion with first and second sections that form first and second segments of the dielectric transmission line. The second segment matches the first segment to the waveguide. It may have a greater width (lesser impedance) than the first section or a narrower width (greater impedance) in order to effect a match according to whether a capacitive or inductive matching structure is required.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of copending U.S. ProvisionalApplication Serial No. 60/251,564 filed Dec. 7, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Technical Field

[0003] This invention relates generally to radio frequency (RF) circuitsoperating at millimeter-wave frequencies, and more particularly to atransmission line to waveguide mode transformer that can be convenientlyimplemented using printed circuit board (PCB) techniques.

[0004] 2. Description of Related Art

[0005] Numerous commercial millimeter-wave systems require transmissionline to waveguide mode transitions. The key requirements of such atransition are that it be compact in size, easy-to-manufacture, andprocess-tolerant for PCB compatible fabrication. Additionally, goodelectrical isolation is desired when multiple transitions from asubstrate are required for a multi-beam system, such as, for example, anautonomous cruise control (ACC) system.

[0006] A number of such transitions have been proposed in theliterature. The well-known transitions from microstrip to rectangularwaveguide mode are the E-field probe method and the “ridge waveguidetransition.” Both of them require modification to the waveguide and theyput restrictions on the planar circuit design.

[0007] Recently, a number of transitions compatible with millimeterintegrated circuits (MMIC) and microwave integrated circuits (MIC)processing have been proposed. While these new transitions provide asuitable technology for large-scale, low-cost manufacturing, theperformance of the printed transition has been unsatisfactory in somerespects. First, they have excessive loss (up to 2 dB) and/or arelatively narrow bandwidth. Second, the transitions are prone to crosstalk and they are sensitive to manufacturing tolerances and operatingenvironment.

[0008] U.S. Pat. No. 6,087,907 issued Jul. 11, 2000 to the inventor ofthe instant application, described a transverse electric orquasi-transverse electric mode to waveguide mode transformer using finsto provide a microstrip transmission line to rectangular waveguide modeconversion. The transition is tolerant of process and provides excellentbroadband performance. In addition, since it does not rely on resonancephenomenon for coupling, it provides excelled isolation between twoadjacent transitions. However, the bandwidth of the transition islimited and the fins occupy area and need good lithography. Thus, thereremains a need for a transmission line to waveguide mode transformer formillimeter-wave frequency modules that is easy to manufacture, provideslow loss, is tolerant to manufacturing variations, and exhibits agreater bandwidth.

SUMMARY OF THE INVENTION

[0009] This invention addresses the need outlined above by providing atransmission line to a waveguide mode transformer with a matchingstructure that improves bandwidth. Described subsequently with referenceto a microstrip to rectangular waveguide (MS-to-RW) mode transformer, orjust transformer, it achieves the improved bandwidth with a matchingsection of the transmission line at the waveguide. The matching sectionis sized and shaped according to known techniques so that it has desiredmatching characteristics, thereby resulting in an ultra-broadbandtransition that does not rely on resonance phenomenon for coupling.

[0010] Thus, the transformer has significantly greater broadbandperformance. It exhibits excellent isolation between two adjacenttransitions. It is process tolerant. It is readily fabricated with knownPCB technology.

[0011] To paraphrase some of the more precise language appearing in theclaims, a transmission line to waveguide mode transformer constructedaccording to the invention includes a dielectric substrate and anelectrically conductive pattern on the substrate (e.g., deposited,evaporate, or rolled laminate). The electrically conductive patternincludes a waveguide potion that forms a dielectric waveguide, and atransmission line portion that forms a dielectric transmission lineextending to the waveguide. The transmission line portion of the patternincludes first and second sections. The first section forms a firstsegment of the transmission line removed from the waveguide, while thesecond section forms a second segment or matching segment of thetransmission line extending from the first segment to the waveguide. Thesecond section of the transmission line portion of the pattern has asize and shape resulting in the second segment of the transmission linehaving characteristics that help match the first segment of thetransmission line to the waveguide.

[0012] The first section of the transmission line portion of theelectrically conductive pattern has a width resulting in a desiredtransmission line impedance (e.g., 50 Ohms). Depending on the design,the second section of the transmission line portion of the pattern mayhave a width that is wider than the first section so that the secondsegment of the transmission line has lower impedance than the firstsegment, or the second section of the transmission line portion of thepattern may have a width that is narrower than the first section so thatthe second segment of the transmission line has higher impedance thatthe first segment in order to match the transmission line to thewaveguide. Fins are not necessary, broadbanded performance results, andthe second section of the transmission line portion of the pattern mayhave a nonuniform width and use known PCB-implemented match-improvingmethods with multi-sections (i.e., shutcap-inductor-shutcap, etc.). Thefollowing illustrative drawings and detailed description make theforegoing and other objects, features, and advantages of the inventionmore apparent.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 of the drawings is an isometric view of a transmission lineto waveguide mode transformer constructed according to the invention(not to scale), with the thickness of the electrically conductivepattern exaggerated for illustrative convenience and the orientation ofan X-Y-Z Cartesian coordinate system depicted in the lower right-handcorner;

[0014]FIG. 2 is a slightly enlarged plan view of the transmission lineto waveguide mode transformer (not to scale), with some of the waveguideportion of the pattern broken away to expose the far end portion of thesubstrate;

[0015]FIG. 3 is a cross sectional view of a first section of thetransmission line portion of the pattern (not to scale) as viewed in atransverse plane perpendicular to the Y-axis that contains a line 3-3 inFIG. 2;

[0016]FIG. 4 is a cross sectional view of the second section (matchingsection) of the transmission line portion of the pattern (not to scale)as viewed in a transverse plane perpendicular to the Y-axis thatcontains a line 4-4 in FIG. 2;

[0017]FIG. 5 is a cross sectional view of the waveguide portion of thepattern (not to scale) as viewed in a transverse plane perpendicular tothe Y-axis that contains a line 5-5 in FIG. 2;

[0018]FIG. 6A is a plot of S-parameters S₁₁ and S₂₁ for a MS-to-RW modetransformer without fins;

[0019]FIG. 6B is a plot of S-parameters S₁₁ and S₂₁ for a MS-to-RW modetransformer with one fin;

[0020]FIG. 6C is a plot of S-parameters S₁₁ and S₂₁ for a MS-to-RW modetransformer that uses some of the technology in U.S. Pat. No. 6,087,907with two fins;

[0021]FIG. 6D is a plot of S-parameters S₁₁ and S₂₁ for a MS-to-RW modetransformer that uses some of the technology in U.S. Pat. No. 6,087,907with three fins;

[0022]FIG. 7 is a plot of S-parameters S₁₁ and S₂₁ for the illustratedtransmission line to waveguide mode transformer of the presentinvention, with the scale for S₂₁ magnified by a factor of ten forbetter visualization;

[0023]FIG. 8 is a plot of electromagnetic (EM) simulated and analyticalS-parameters for a transmission line to waveguide mode transitionwithout the matching structure of the present invention, the large dotsindicating data points from the Z_(PI) equation subsequently described;

[0024]FIG. 9 is a plot of an EM-optimized transmission line to waveguidemode transition that includes a low impedance line and a capacitivestub; and

[0025]FIG. 10 is a plan view similar to FIG. 2 showing the pattern of asecond embodiment in which the matching portion of the pattern isnarrower than the first section of the transmission line portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] FIGS. 1-9 of the drawings show various aspects of a transmissionline to waveguide mode transformer 10 constructed according to theinvention. It is similar in some general respects to the transformerdescribed in U.S. Pat. No. 6,087,907 issued Jul. 11, 2000 to theinventor of the instant application that is entitled “TransverseElectric or Quasi-Transverse Electric Mode to Waveguide ModeTransformer.” That patent is incorporated herein by this reference forthe information it provides, including background information,construction and design details, and MS-to-RW mode transitionunderstandings.

[0027] The transformer 10 includes important differences. It will bedescribed in terms of the geometry of a substrate and an electricallyconductive pattern on the substrate. The substrate and electricallyconductive pattern combine to form circuit elements, including awaveguide and a transmission line leading to the waveguide. Focusing onthis geometry in order to introduce the nomenclature developed for thisdescription and the claims, the transformer 10 includes a dielectricsubstrate 11 (FIGS. 1-5) having orthogonal X, Y, and Z axes and a planeof symmetry 12 (FIGS. 2-5) that contains the Y-axis and the Z-axis. Thesubstrate 11 includes a first surface 13 (FIGS. 2-4) containing theX-axis and the Y-axis (and perpendicular to the plane of symmetry), anoppositely facing and spaced-apart second surface 14 (FIGS. 3-4)parallel to the first surface 13, and spaced-apart first and secondsides 15 and 16 (FIGS. 2-5) that extend between the first and secondsurfaces 13 and 14 parallel to and on opposite sides of the plane ofsymmetry 12. This configuration can be achieved with a dielectricsubstrate in the form of a rectangular prism.

[0028] An electrically conductive pattern on the substrate 11 forms adielectric waveguide and a dielectric transmission line extending to thewaveguide. A waveguide portion 17 of the electrically conductive pattern(FIGS. 1, 2, and 5) on the first and second surfaces 13 and 14 and onthe first and second sides 15 and 16, forms the dielectric waveguide ona far end portion 11A of the substrate 11 (FIGS. 2 and 5). The waveguideportion 17 and the waveguide it forms are disposed symmetricallyrelative to the plane of symmetry 12.

[0029] A groundplane portion 18 of the electrically conductive pattern(FIGS. 1, 3, and 4) on the second surface 14 extends from a near endportion 11B of the substrate 11 (FIGS. 2 and 3) to the waveguide portion17. A transmission line portion of the electrically conductive patternon the first surface 13 includes a first section 19 (FIGS. 1, 2, and 3)and a second section 20 (FIGS. 1, 2, and 4). The first section 19extends to a junction 20A with the second section 20 that is depicted inFIG. 2 by a dashed line, and the second section 20 (or matchingstructure) extends to a junction 20B with the waveguide portion 17 thatis also depicted by a dashed line.

[0030] The first section 19 of the transmission line portion of theelectrically conductive pattern extends along the Y-axis from the nearend portion 11B of the substrate 11 toward the waveguide. Combined withthe groundplane portion 18, the first section 19 forms a first segmentof the transmission line. The second section 20 of the transmission lineportion of the electrically conductive pattern (FIGS. 1, 2, and 4)extends along the Y-axis from the first section 19 to the waveguide.Combined with the groundplane portion 18, the second section 20 forms asecond segment (or matching segment) of the transmission line. The firstand second sections 19 and 20 of the transmission line portion of theelectrically conductive pattern, and the first and second segments ofthe transmission line they form, are disposed symmetrically relative tothe plane of symmetry 12.

[0031] The substrate 11 may be composed of Duroid or other suitabledielectric material, Duroid being a proprietary product of RogersCorporation consisting of woven glass/PTFE laminates. The substrate 11has a thickness (i.e., a Z-dimension) measured along the Z-axis, a width(i.e., an X-dimension) measured along the X-axis, and a length (i.e., aY-dimension) measure along the Y-axis. The dimensions may vary accordingto the dielectric material and other design parameters. For purposes ofthis description, the dielectric substrate 11 is composed of Duroid 5880with a dielectric constant of 2.2. It has a thickness of 127 microns, awidth of 2020 microns, and a length that is determined by theapplication. With the waveguide portion 17 of the electricallyconductive pattern covering the substrate 11 to form the dielectricwaveguide, the waveguide has a thickness or Z-dimension equal to the127-micron thickness of the substrate 11 and a width or X-dimensionequal to the 2020-micron width of the substrate 11.

[0032] The first section 19 of the transmission line portion of thepattern has a width or X-dimension of 380 microns. That results in thefirst segment of the transmission line it forms having a 50-Ohmcharacteristic impedance. In addition, the first section 19 has a lengthor Y-dimension (determined by the particular application) that extendsfrom a first port 1 (identified in FIG. 2) toward the waveguide portion17, while the waveguide portion 17 has a length or Y-dimension(determined by the particular application) that extends from the secondsection 20 to a second port 2 (identified in FIG. 2). RF energy iscoupled to or from the transformer 10 via the first and second ports 1and 2. The RF energy propagates within the substrate 11 along theY-axis.

[0033] The second segment of the transmission line formed by the secondsection 20 of the transmission line portion of the pattern serves thefunction of matching the transmission line to the dielectric waveguidefor efficient energy transfer as reflected by the S-parameters discussedlater in this description. To achieve that function, the second section20 has a width or X-dimension different from the X-dimension of thefirst section 19 and a length or Y-dimension that combines with theX-dimension to result in characteristics for the second segment of thetransmission line that achieve the desired impedance transformation.

[0034] The foregoing is a broad statement of the geometry of thetransformer 10 and the second or matching segment of the transmissionline. No fins are required. Low-loss, broadband performance results.According to one preferred embodiment, the second section 20 of thetransmission line portion of the electrically conductive pattern (thematching structure) is wider than the first section 19 so that itintroduces capacitance. In other situations, a narrower, inductivematching structure may be required.

[0035] The second section 20 of the transmission line portion of thepattern has a length or Y-dimension that the microwave PCB designerfactors in with its width or X-dimension to achieve characteristics thatprovide a better match and energy transfer between the transmission lineformed by the transmission line portion of the electrically conductivepattern (i.e., sections 19 and 20) and the waveguide formed by thewaveguide portion 17. Based upon the foregoing and subsequentdescriptions, one of ordinary skill in the art can readily design andfabricate a matching structure according to the invention to achievebroadband performance exceeding that of fin-line designs in U.S. Pat.No. 6,087,907. The matching section may be designed in EMPIPE 3D (atrademark of Agilent Corporation). It is an electromagnetic (EM)optimization tool available from Agilent Corporation that uses MFSS (atrademark of Agilent Corporation) to conduct finite element method (FEM)based EM simulations. The design of the matching section begins with aS-parameter plot on a Smith Chart. For the transformer 10, the SmithChart shows that the element needed to match the structure is acapacitance at the junction 20B (FIG. 2). The X-dimension andY-dimension of the section 20 are chosen through EMPIPE 3D to provide anoptimum match.

[0036] Concerning transition performance, consider a rectangularwaveguide when it is above cut-off. The rectangular waveguide can bethought of as many quarter wavelength strips (fins) extending from thecenter region and shorted at the far end (rectangular waveguide sidewall). The central region of the rectangular waveguide can beapproximated by a parallel-plate waveguide, provided it is abovecut-off. To gradually transfer a microstrip transmission line to theparallel-plate waveguide, one needs fins. The fins help restrict theE-field within the substrate where the microstrip ends. Because arectangular waveguide can also be equated to closely spaced fins, thetransfer of E-field into the substrate is very good, even under no-finconditions. Therefore, a broadband match for the transition is achievedwhen the impedance of the resulting parallel-plate line is approximatelymatched to the microstrip transmission line at the frequency ofinterest.

[0037] FIGS. 6A-6D show simulated results of a MS-to-RW mode transitionimplemented in 127-micron thick Duroid. The microstrip transmission lineis nominally 50-Ohms and is 380 microns wide, while the rectangularwaveguide width is 2020 microns. The S-parameters are in the naturalimpedance of the transmission line and, thus, reflect thecharacteristics of the mode transformer. Simulations show that theradiation loss at the transition is about 0.15 dB per transition and theoperating bandwidth is from 65 GHz to 90 GHz. Also shown in FIGS. 6A-6Dis the result of reducing the number of fingers. It is clear that thenumber of fins can change the frequency where the optimum match isachieved.

[0038] Better performance is obtained by using no fins. For the no-fincase, introducing a slight capacitance at the junction can optimize theperformance for the stated substrate dimensions. Other structures mayrequire an inductive element. FIG. 7 shows the EM-optimized performancewith the S-parameters referred to the natural impedances of thetransmission line. A better than 15 dB return loss is obtained for 60GHz to 140 GHz. bandwidth. The patch at the junction implements thematching capacitance. It measures 342 microns long and 655 microns wide.With different dimensions or material, an inductive (or narrow) line maybe more suitable. Thus, instead of fins, different matching structurescan be used.

[0039] The transition of this invention provides improved and easiermatching that can be used for many differing applications. The baseplate of a millimeter-wave package can have multi-section rectangularwaveguide sections, for example, that can transform the waveguideimpedance into free-space impedance and in the process turn it 90degrees. The resultant transmission line to waveguide mode transition isprocess insensitive. It enables the mounting of MMICs on top of a Duroidsubstrate for millimeter-wave applications even when line lithographymay not be very well controlled. In addition, the transition is usablefor broadband packages, couplers, splitters, and transitions. For easeof manufacturing, the edge wall can be replaced by continuous ground viamimicking the continuous ground wall. Via are plated holes through thesubstrate 11. For some other substrates (e.g., GMIC from M/A-com), theyare solid mesa that bring grounds up the surface. Thus, via areconnections between surfaces 13 and 14, usually accomplished by a holeor a solid pillar. Inasmuch as the transition of this invention allowseasy PCB-based MS-to-RW mode transition, it enables waveguide filters inthe substrate to be built where via could be used as inductive and/orcapacitive elements

[0040] Concerning design procedures, a broadband match for thetransmission line to waveguide mode transition is achieved when theimpedance of the transmission line is matched to the waveguide at thefrequencies of interest. Since the electric current is continuous acrossthe transition, the Z_(PI) (power-current impedance) is the appropriateimpedance to calculate using the following equation:$Z_{PI} = {465\frac{b}{a\sqrt{ɛ_{r}}}\frac{1}{\sqrt{1 - \left( \frac{f_{c}}{f} \right)^{2}}}}$

[0041] where “a” represents the width or X-dimension of the dielectricwaveguide formed by the waveguide portion 17 of the pattern and “b”represents the thickness or Z-dimension, while “ε_(r)” is the relativedielectric constant of the dielectric substrate 11 and f_(c) is thecut-off frequency of the waveguide. From this equation and a microstriptransmission line impedance of 50 Ohms, the return loss for thetransmission line to waveguide mode transformer 10 is obtained. Thebandwidth of the transformer 10 is determined by considering the TE_(N0)modes. For a practical region of interest, N=1,2,3 are the mostimportant modes. The cut-off frequencies of these modes are spaced in aratio of 1:2:3. The TE₂₀ mode is orthogonal to the microstrip mode andis not excited at a symmetric junction. So, the transformer between theMS-to-TE₁₀ mode is restricted once the TE₃₀ mode is excited.

[0042] In practical situations, the dielectric substrate ispredetermined due to a number of reasons such as microstrip losses,device implementation, packaging issues, and substrate availability. Apopular PCB design substrate for mm-wave applications is RT/Duroid 5880with a dielectric constant of 2.2 and a thickness of 127 microns. Onthis substrate, if the cut-off frequency of the TE₁₀ mode is chosen tobe 50 GHz, then “b” is calculated to be 2020 microns. FIG. 8 shows theAnsoft HFSS simulated S-parameters between the transmission line (port1) and various modes on the dielectric waveguide (port 2) for aone-millimeter long microstrip transmission line (50-Ohm nominally −380microns wide) and a one-millimeter long rectangular dielectricwaveguide. Ansoft HFSS is a registered trademark of Ansoft Corporationof Pennsylvania. Also plotted are the S-parameters based on theimpedance definition given in the equation for Z_(PI) stated above.

[0043] Thus, this theory clearly predicts the return loss. Theexcitation of the TE₂₀ mode is low across the entire frequency band.Breakpoints in the TE₃₀ mode at about 100 GHz and in the TE₃₀ mode at150 GHz correspond to the rectangular waveguide cut-off frequencies.Since the impedance of the waveguide is smaller than that of thetransmission line, a stepped impedance transformer with simple matchingstructure are implemented in the EM simulator to match the transition.

[0044]FIG. 9 shows an EM-optimized match for the transmission line towaveguide mode transformer 10. Better than 18-dB return loss is obtainedover a 60-140 GHz bandwidth, showing the broadband nature of thetransition. The match is sufficiently good for most practicalapplications.

[0045] As mentioned previously, the second or matching segment of thetransmission line may be wider than the first section (capacitive) ornarrower (inductive). FIG. 10 illustrates the inductive case with atransformer 100. The transformer 100 is similar in many respects to thetransformer 10 and so only differences are described in further detail.For convenience, numerals designating parts of the transformer 100 areincreased by one hundred over those designating corresponding, similar,or related parts of the transformer 10.

[0046] The transformer 100 includes an electrically conductive patternon a substrate 111. A waveguide portion 117 of the pattern forms adielectric waveguide. A transmission line portion of the patternincludes first and second sections 119 and 120 that form a transmissionline extending to the waveguide. The first section 119 extends to ajunction 120A with the second section 120, and the second section 120extends to a junction 120B with the waveguide portion 117, with thesecond section 120 being narrower than the first section (i.e.,inductive). As with the transformer 10, multi-stage matching may beused.

[0047] Thus, the invention provides a transmission line to waveguidemode transformer with an ultra-broadband transition that does not relyon resonance phenomenon for coupling. The transformer exhibits excellentisolation between two adjacent transitions, it is process tolerant, andit is readily fabricated with known PCB technology. Although exemplaryembodiments have been shown and described, one of ordinary skill in theart may make many changes, modifications, and substitutions withoutnecessarily departing from the spirit and scope of the invention. Forexample, the matching section of the transmission line may be ofnonuniform width along a portion of its length and such a configurationis intended to fall within the scope of the broader claims that follow.

What is claimed is:
 1. A transmission line to waveguide modetransformer, comprising: a substrate; and an electrically conductivepattern on the substrate that forms a waveguide and a transmission lineextending to the waveguide; the electrically conductive patternincluding a waveguide portion and a transmission line portion extendingto the waveguide portion; the transmission line portion of the patternincluding a first section that forms a first segment of the transmissionline removed from the waveguide; and the transmission line portion ofthe pattern including a second section that forms a second segment ofthe transmission line extending from the first segment of thetransmission line to the waveguide, which second segment hascharacteristics that help match the first segment to the waveguide.
 2. Atransmission line to waveguide mode transformer as recited in claim 1,wherein the second section of the transmission line portion of thepattern is wider than the first section so that the second segment ofthe transmission line has lower impedance than the first segment.
 3. Atransmission line to waveguide mode transformer as recited in claim 1,wherein the second section of the transmission line portion of thepattern is narrower than the first section so that the second segment ofthe transmission line has a higher impedance than the first segment. 4.A transmission line to waveguide mode transformer as recited in claim 1,wherein the transmission line is a microstrip transmission line and thewaveguide is a rectangular waveguide.
 5. A transmission line towaveguide mode transformer as recited in claim 1, wherein the secondsection of the transmission line portion of the pattern has a uniformwidth.
 6. A transmission line to waveguide mode transformer as recitedin claim 1, wherein: the first section of the transmission line portionof the pattern has a first width; the second section of the transmissionline portion of the pattern has a second width; and the second width isdifferent from the first width for at least a portion of the secondsection.
 7. A transmission line to waveguide mode transformer as recitedin claim 1, wherein: the first section of the transmission line portionof the electrically conductive pattern has a first width; and the secondsection of the transmission line portion of the electrically conductivepattern has a second width different from the first width and a lengthresulting in desired matching characteristics.
 6. A transmission line towaveguide mode transformer, comprising: a substrate having orthogonal X,Y, and Z axes, a plane of symmetry containing the Y and Z axes, andparallel first and second surfaces perpendicular to the plane ofsymmetry; and an electrically conductive pattern on the substrate thatforms a waveguide disposed symmetrically relative to the plane ofsymmetry and a transmission line extending to the waveguide; theelectrically conductive pattern including a waveguide portion and atransmission line portion that extends to the waveguide portion; thetransmission line portion of the pattern including a first sectionextending along the Y-axis that forms a first segment of thetransmission line; and the transmission line portion of the patternincluding a second section extending along the Y-axis from the firstsection to the waveguide portion that forms a second segment of thetransmission line, which second section of the transmission line portionof the pattern has a size and shape resulting in desired matchingcharacteristics.